Our overall goal is to determine the manner by which the peptidyl-prolyl isomerase, Pin1, acts as a negative regulator of mitosis and evaluate whether this protein may be an attractive target for development of an anti-cancer therapeutic agent. Pin1 was discovered as a protein that interacts with NIMA, a protein Ser/Thr Kinase that is essential for progression from G2 to mitosis in Aspergillus nidulans. Pin1 has been conserved from yeast to man and may be critical for mitotic progression. We cloned Pin1 from Asperigillus and Xenopus, and made antibodies to the protein. We showed that Pin1 will regulate entry into mitosis in Xenopus extracts where it interacts with key components of the p34cdc2 regulatory pathway such as the active forms of the cdc25 protein phosphatase and the plxl protein kinase as well as a number of other mitotic phosphoproteins recognized by the MPM2 antibody. We propose to use the Xenopus Egg/oocyte system to address key issues regarding the prolyl isomerase and protein binding of Pin1 and elucidate critical details of its role(s) as a cell cycle regulator. We will evaluate the tole of Pin1 in the timing of mitotic entry and in the operation of checkpoints governing the G2/M transition as well as examine if Pin1 plays a role in exit from mitosis. In parallel, we will determine if Pin1 is essential for mouse development and focus on its role in development, activation and/or proliferation of T lymphocytes by using the cre/lox system to disrupt the Pin1 gene in mice either globally or specifically in T lymphocytes. We will determine if phenotypic consequences of Pin1 deletion in either vertebrate system requires the prolyl isomerase activity. Completion of these studies will define the role(s) of Pin1 in growth and development, determine its critical cell cycle functions and clarify whether the Pin1 prolyl isomerase is a good candidate for anti-cancer drug development.